1. Field of the Invention
The present invention relates to a photoelectric conversion device in which a single crystal semiconductor or a polycrystalline semiconductor is used, and a photoelectric conversion device in which a plurality of photoelectric conversion elements are stacked.
2. Description of the Related Art
Photovoltaics is spreading as a clean and inexhaustible energy source. For photovoltaics, a photoelectric conversion device (also referred to as a “solar battery”) which converts photo energy into electrical energy with use of photoelectric characteristics of a semiconductor is used.
The production of photoelectric conversion devices is increasing every year. For example, the worldwide production amount of solar batteries in 2005 is 1759 MW, which means drastic increase of 147% in comparison with the previous year. The photoelectric conversion devices using a crystal semiconductor is spreading worldwide, and photoelectric conversion devices using a single crystal silicon substrate or a polycrystalline silicon substrate account for a large part of the production amount.
As the demand for crystalline photoelectric conversion devices using silicon as a material increases, problems such as short supply of polycrystalline silicon which is a material of a silicon substrate and escalating price of polycrystalline silicon due to the short supply occur. The production amount of polycrystalline silicon in 2007 is estimated to be about 36,000 ton, and on the other hand, polycrystalline silicon of 25,000 ton or more is needed for a semiconductor (LSI) and 20,000 ton or more is needed for solar batteries; therefore, shortage of about 10,000 ton is expected. Such supply shortage is expected to continue.
The adequate thickness of a crystalline photoelectric conversion device using silicon as a material is 10 μm so that the crystalline photoelectric conversion device can absorb solar light. On the other hand, a single crystal silicon substrate or a polycrystalline silicon substrate which serves as a base material of the crystalline photoelectric conversion device has a thickness of about 200 μm to 300 μm. Thus, the photoelectric conversion device using the single crystal silicon substrate or the polycrystalline silicon substrate has a thickness which is more than 10 times as thick as the thickness needed for photoelectric conversion. Supply shortage of silicon substrates also results from inefficient use of expensive semiconductor materials by photoelectric conversion devices.
Photoelectric conversion devices have various structures. As well as photoelectric conversion devices having a typical structure in which an n-type or p-type diffusion layer is formed over a single crystal silicon substrate or a polycrystalline silicon substrate, stacked-layer photoelectric conversion devices having a structure in which different unit cells, which are a unit cell formed of a single crystal semiconductor and a unit cell formed of an amorphous semiconductor, are combined are known (e.g., see Patent Document 1: Japanese Published Patent Application No. H6-44638).
The stacked-layer photoelectric conversion devices in which a plurality of photoelectric conversion unit cells are stacked have problems in that a junction in an opposite direction to the unit cells (opposite junction) is formed at a junction portion of an upper-layer unit cell and a lower-layer unit cell and current does not flow well, and thus output characteristics of the photoelectric conversion device are decreased. As a solution of such problems, a technique is known in which a metal thin film, a silicide film, or the like is interposed to solve the opposite junction to form an ohmic contact (e.g., see Patent Document 2: Japanese Published Patent Application No. H1-47907, Patent Document 3: Japanese Published Patent Application No. H5-25187, and Patent Document 4: Japanese Published Patent Application No. H5-43306).